An increase in circulating autoantibodies in aging humans, documented in many studies, is one of several changes in B cell and T cell function that may signal immune senescence. This senescence, in turn, may underlie increased susceptibility to infectious disease and malignancy in the aged. The proposed research aims to test possible mechanisms that may contribute to autoantibody formation in aging, distinguishing between: i) continued or increased production of natural autoantibodies encoded by V region gene segments with few or no mutations and ii) production of autoantibodies with mutated V region segments, characteristic of antigen-selected antibodies and/or accumulation of random mutations. Healthy volunteers will be screened to identify three groups of subjects: i) elderly (over 65 years) with autoantibodies to at least one of a panel of autoantigens; ii) elderly without the autoantibodies; and iii) young adults without the autoantibodies. Plans are to test isotypes, cross-reactivity, and relative affinities of autoantibodies in the positive sera and expression of a recurrent idiotype. Comparing the three subject groups, the investigators will: i) analyze EBV-transformed B cells to determine the fraction of transformed cells producing autoantibodies, their isotypes, and cDNA sequences for H and L chain V regions of Ig from autoreactive and non-autoreactive clones. ii) probe V region cDNA libraries from peripheral blood B cells to determine the frequency of use of: VH families, VK families, or individual V gene segments that are prominent in the natural autoantibody and normal repertoire of young persons. They will sequence the V regions of randomly chosen clones and clones with recurrent VH and VK segments to determine the frequency, distribution and coding effects of mutations in FR and CDR sequences. iii) analyze libraries of rearranged H chain and K chain V region DNA from peripheral blood B cells in the same way as the cDNA libraries as a second test of repertoire and the frequency, site and nature of mutations, including those in nonproductive rearrangements.